It is well known that ethylene (and α-olefins in general) can be readily polymerized at low or medium pressures in the presence of certain transition metal catalysts. These catalysts are generally known as Ziegler-Natta type catalysts.
A particular group of these Ziegler-Natta type catalysts, which catalyse the polymerization of ethylene (and α-olefins in general), comprise an aluminoxane activator and a metallocene transition metal catalyst. Metallocenes comprise a metal bound between two η5-cyclopentadienyl type ligands. Generally the η5-cyclopentadienyl type ligands are selected from η5-cyclopentadienyl, η5-indenyl and η5-fluorenyl.
It is also well known that these η5-cyclopentadienyl type ligands can be modified in a myriad of ways. One particular modification involves the introduction of a linking group between the two cyclopentadienyl rings to form ansa-metallocenes.
Numerous ansa-metallocenes of transition metals are known in the art. However, there remains a need for improved ansa-metallocene catalysts for use in polyolefin polymerization reactions. In particular, there remains a need for new metallocene catalysts with high polymerization activities/efficiencies.
There is also a need for catalysts that can produce polyethylenes with particular characteristics. For example, catalysts capable of producing linear high density polyethylene (LHDPE) with a relatively narrow dispersion in polymer chain length are desirable.
WO2011/051705 discloses ansa-metallocene catalysts based on two η5-indenyl ligands linked via an ethylene group.
There remains a need for ansa-metallocene catalysts having improved polymerization activity. Moreover, due to the high value that industry places on such materials, there is also a need for ansa-metallocene catalysts capable of polymerizing α-olefins to high molecular weights, without compromising polydispersity. It is even further desirable that such catalysts can be easily synthesized.
The present invention was devised with the foregoing in mind.